This invention relates to a fuel injection pump, and more particularly to an improved cam for use in a fuel injection pump of the distribution type.
As is well known, a fuel injection pump of the distribution type comprises a drive shaft and a cylinder which are mounted on a housing in coaxial relation to each other. One end portion of a plunger is received in the cylinder, and cooperates therewith to form a pump chamber. Within the housing, the other end of the plunger is disposed in opposed relation to one end of the drive shaft. The rotational movement of the drive shaft is converted by a cam mechanism into axial reciprocal movement and rotational movement of the plunger. Fuel in the pump chamber is pressurized by the advance stroke of the axial reciprocal movement of the plunger, and the fuel is drawn into the pump chamber by the return stroke of this reciprocal movement. Each time the fuel in the pump chamber is pressurized, the pump chamber is sequentially connected, through the rotation of the plunger, to a plurality of (for example, four) delivery valves, mounted on the housing, via a passage formed in the plunger. As a result, injection nozzles connected respectively to the delivery valves sequentially inject the fuel to cylinders of an engine, respectively.
The above cam mechanism comprises a plurality of (for example, four) rollers supported on the housing, a cam disposed in opposed relation to the rollers, and a spring urging the cam toward the rollers. The cam is connected to the one end of the drive shaft in such a manner as to transmit the rotation of the drive shaft to the cam and also to allow the cam to move axially. The other end of the plunger is connected to the cam in such a manner as to transmit the rotation of the cam to the plunger and also to cause the plunger to move axially together with the cam.
The surface of the cam facing the rollers serves as a cam surface. A plurality of (for example, four) mountain-like cam projections of identical shape are formed on the cam surface at equal intervals in the direction of the periphery of the cam. During the rotation of the cam, when the roller is disposed at a lift region extending from a leading end of the cam projection to a peak thereof, the cam is lifted in a direction away from the roller to move or advance the plunger. When the roller is disposed at a descend region extending from the peak of the cam projection to a trailing end thereof, the cam descend in a direction toward the roller to return the plunger.
The design of the cam projection must meet the following requirements:
(a) The fuel must be injected under high pressure. With this high-pressure injection, the fuel injection rate (i.e., the amount of injection of the fuel per unit time) can be increased, thereby reducing the amount of production of Nox and smoke. The high-pressure injection can be achieved by increasing the maximum speed of advance stroke of the plunger, that is, the maximum lift speed of the cam.
(b) The maximum lift amount of the cam must be limited. If the maximum lift amount is increased, the resilient deformation of the spring urging the cam is increased, which results in a shortened lifetime of the spring.
(c) The pressure of contact between the cam surface and the roller must be kept to a low level. By doing so, the lifetime of the cam surface can be prolonged.
Japanese Laid-Open (Kokai) Utility Model Application No. 95570/89 shows in FIG. 5 the relation between a lift speed of a cam and a cam angle. A lift region of a mountain-like cam projection has a first angle portion where the lift speed of the cam linearly increases relatively abruptly, a second angle portion where the lift speed linearly decreases relatively gently, and a third angle portion where the lift speed linearly decreases relatively abruptly. The maximum value of the lift speed appears at the boundary between the first and second angle portions. The first angle portion has a concavely curved surface, and each of the second and third angle portions has a convexly curved surface.
In the above prior publication, when it is intended to meet the requirement (a) quite satisfactorily, the other requirements (b) and (c) fail to be met. Namely, if the maximum lift speed is made higher than that shown in FIG. 5 of the above prior publication, the lift speed at the second angle portion is increased, and hence the maximum lift amount which is the integral value of the lift speed is increased, so that the requirement (b) fails to be met.
In view of the above, if the maximum lift speed is made higher than that shown in FIG. 5 of the above prior publication, and at the same time the degree of decrease of the lift speed (i.e., the deceleration) at the second angle portion is made greater, then the maximum lift amount can be controlled to an acceptable level. In this case, however, the requirement (c) can not be met, because if the deceleration is increased, the radius of curvature of the second angle portion is decreased, so that the area of contact between the roller and the second angle portion is decreased. As a result, the pressure of contact between the second angle portion and the roller which is produced by the resilient force of the spring and the pressure in the pump chamber increases, which results in a shortened lifetime of the second angle portion.